Telomeres are unique terminal chromosomal structures which shorten with cell division in vitro and with increased age in vivo for human somatic cells. Telomerase, a ribonucleoprotein enzyme that is capable of synthesizing telomeric repeats, is expressed in germline and malignant cells and is absent in most normal human somatic cells. When telomerase activity was analyzed in normal human lymphocytes, it was found that telomerase is expressed during development and activation of T and B lymphocytes, identifying a potential role for this enzyme in maintaining the capacity of lymphoid cells for clonal expansion. Expression of the two genes encoding the necessary and sufficient components of telomerase, RNA template (TR) and reverse transcriptase catalytic component (TERT), was analyzed, and both mRNAs were found to be regulated during lymphocyte development and activation. To further study the regulation of telomerase at a transcriptional level, a series of genetically engineered mice have been constructed including: mouse TERT (mTERT) cDNA transgenics, GFP knock-in as a reporter for mTERT transcriptional activity (also resulting in mTERT knock-out inactivation), and a human TERT (hTERT) bacterial artificial chromosome (BAC) transgenic that allows study of human TERT regulation in human versus mouse cellular environments. It has been observed that in vivo expression of human telomerase differs significantly from that of mouse telomerase. To assess the basis for this difference, a BAC clone containing the entire hTERT gene was introduced in mice. In these transgenic mice, hTERT transgene expression was similar to endogenous hTERT in humans, and different from endogenous mTERT (mouse TERT): in tissues showing a significant difference in expression levels between hTERT in humans and mTERT in mice, hTERT transgene expression was repressed, mimicking hTERT expression in humans. Reporter constructs and mutational analysis of the hTERT and mTERT promoters revealed that a non-conserved GC-box within the hTERT promoter was responsible for the human-specific repression. Transfection experiments also demonstrated that the GC-box-mediated a human-specific mechanism for TERT repression is impaired in human cancer and may play a role in immortalization and malignant transformation. These results reveal that a difference in cis-regulation of transcription, rather than trans-acting transcription factors, is critical to the difference in TERT tissue-specific expression between species. This insight into species-specific regulation of telomerase and telomeres provides an experimental basis for generating mice humanized for telomerase enzyme and its pattern of expression, a model that may be useful in studying both normal human biology and pathologic processes including malignant transformation. To translate these findings to models of clinical relevance, we have studied the significance of telomere length maintenance in clinical cancer immunotherapy, in tumor-specific populations of tumor-infiltrating lymphocytes (TILs) that are isolated from human tumors, expanded in vitro, and used to treat the donor patients. The hypothesis underlying these studies was that telomere length will predict in vivo persistence of T cells used in therapy as well as their clinical effectiveness. Results indicate that telomere length of administered TILs is highly correlated with persistence of TILs after administration to patients, and is correlated as well with survival of patients treated for malignant melanoma. TILs fail to express significant telomerase activity either in vitro or after in vivo administration to patients. These findings provide a means for predicting effectiveness of treatment in individual patients as well as a basis for enhancing therapeutic effect by interventions designed to maintain telomere length.